Burnett River Floods and Gauging & Rating Implications

Ray Maynard - Qld Dept of Natural Resources and Mines , 30 October, 2014

The Burnett and surrounding catchments had large floods in December 2010, the highest at Bundaberg since February 1942, and peak discharges were as high as 9850 cumecs. In late-January 2013 the same area had record or near-record floods after 72 h rainfalls of up to 1372 mm from ex Tropical Cyclone Oswald, with peak discharges of up to 17 000 cumecs. The 2010-11 and 2013 floods are discussed, including the meteorological situation, rainfall, historic flood levels, flood peak discharges and runoff, flood warning liaison, Department of Natural Resources and Mines (DNRM) flood damage and repairs, and post-floods elevated water conductivity.

Innovative gauging techniques were used by Bundaberg hydrographers to gauge flows up to 7329 cumecs, due to high water velocities of up to 6.57 m.s-1. Their 2010-11 and 2013 flood gaugings are briefly described, and suggestions are given for enhancing Acoustic Doppler Current Profiler (ADCP) gaugings and data processing, and for ADCP system improvements. Gauging uncertainty sources such as differences between ADCP and current meter gaugings, ADCP moving-bed adjustments, gauging reach-travel-time adjustments, gauging reach-storage-change adjustments, gauging unsteady-flow adjustments, and gauging height datum uncertainty are discussed. The emphasis is on ensuring that uncertainty is from random errors, rather than from systematic bias errors.

Multiple rating techniques have been used to extrapolate numerous rating curves above the 2013 flood level. Those discussed are straight GH-Q power law rating lines, power law relationships for area or velocity or resistance, typical rating exponents, rating segments, single-section rating extrapolations, slopes differentiated from bed profiles, and RTK cross section surveys. Rating uncertainty sources such as uncertain Gauckler-Manning n values, vegetation-induced rating changes, loop rating effects (unsteady flow), floodplain flows, other site-specific rating anomalies, peak Q-Catchment area power law scaling, flood event water-balances, flood warning model ratings, and rainfall variation with height and position are also discussed.